Light

Question 1

Luminance

Answer 1

The photo metric measure of the brightness of a lit surface

Question 2

Reflectance

Answer 2

The ratio of reflected light to incident light at an opaque surface

Question 3

Transmittance

Answer 3

The ratio of transmitted light to incident light for a translucent surface

Question 4

Illuminance

Answer 4

Light reaching a surface

Lux/lumens/m^2

Question 5

The no sky line

Answer 5

Method of assessing penetration of daylight
Works if little external obstruction
The line on the working plane beyond which no direct light from the sky will penetrate

Question 6

Daylight factor

Calculate

Importance (especially in U.K.)

Answer 6

Ratio of the illuminate at a point within a building to the illuminate at an unobstructed horizontal surface outside at the same time. Percentage

Df= (li/lo) x100%

Important in uk as we have most overcast days but actual level of daylight illuminate can vary massively with time and weather conditions. Daylight factor useful for diffuse/overcast conditions

Question 7

Daylight arriving at a building has three components SC, ERC, IRC

Answer 7

Sky component -arrives from unobstructed sky
ERC - sky light that arrived after reflection from external surfaces
IRC - sky light that arrived after reflection from internal surfaces

Question 8

Df of a rectangular room with vertical windows

Answer 8

Df ave = (T.W.õ.M) / (A.(1-R^2)) %

T is diffuse transmittance of glazing
W is net area in m2 of windows (glass area)
Õ is angle in vertical plane of visible sky from centre of window
M is maintenance factor to allow for dirt on window
A is total area in m2 of interior surfaces (inc Windows)
R is average reflectance of interior surfaces (~0.5)

Question 9

If df is:

5% or more

2-5%

Below 2%

Answer 9

Daytime electric lighting is usually unnecessary

Daylit appearance but electric lighting is usually necessary in working interiors to 1. Enhance illuminances on surfaces distance from window 2. Reduce contrast with view outside

Electric lighting is necessary and appears dominant. Window provided exterior view but gives only local lighting

Question 10

Light flux

Answer 10

Rate of flow of light energy as perceived by the eye (lumen)

Question 11

Glare

Answer 11

The discomfort or impairment of vision caused by excessive brightness range in the visual field